Optical disks such as magneto-optic recording disks include recording layers as information carrying means on disk substrates. In the case of double side recording type, a pair of disk substrates are mated such that the recording layers are sandwiched between the substrates for the purpose of protecting the recording layers against mechanical damage as by scratching.
In bonding paired disk substrates for fabricating integral optical disks, there are used a variety of adhesives including thermal fusion type adhesives such as hot-melt adhesives, reaction type adhesives such as epoxy adhesives and UV-curable adhesives, and pressure-sensitive adhesives or self-adhesives. Usually, such bonding operation is carried out in clean room conditions for the purpose of preventing any contamination of optical disks, for example, by dust and debris existing between disk substrates. The use of hot-melt adhesives requiring heat is not recommended for clean room working. Epoxy adhesives and UV-curable adhesives require a relatively long time to complete bonding and leave the risk that curing agents and sensitizers in the adhesives can cause corrosion of recording layers or information carrying means. For this reason, self-adhesives are advantageous since they are free of the detrimental action and feasible to clean room working.
Use of self-adhesives, however, has some problems. In applying self-adhesive to a disk substrate, air can be confined between the disk substrate and the self-adhesive layer. Such air bubbles causes a lowering of disk substrate bond strength particularly when the substrates are of resin. The completed disks can be locally increased in thickness where air bubbles are entrained, resulting in a loss of mechanical precision or dimensional accuracy. Then the disks will experience a substantial amount of axial runout when rotated, leading to difficult focusing and error occurrence.
It is thus necessary to avoid entrainment of air bubbles by applying self-adhesive in vacuum in mating disk substrates together. Alternatively, air bubbles are removed by high temperature baking. Attachment in vacuum and high temperature baking add complexity or an extra step to the optical disk manufacturing process, and are disadvantageous for mass production.
Where self-adhesive compositions and hot-melt adhesive compositions are used, a pair of disk substrates are joined into an integral disk by applying an adhesive layer on the inside surface of each disk substrate, closely mating the disk substrates through the adhesive layers, and pressing the assembly by means of a pressing machine. If the inside surface or bonding surface of one disk substrate is deformed to present irregularities, the pressing would cause the other disk substrate be deformed in conformity to the irregularities. The resulting disk becomes low in mechanical or dimensional precision and thus experiences noticeable runout upon rotation.